Which one would be louder?

Even without knowing all numbers I'm pretty sure choice B would be louder. The SVT4 has nearly four times the power, and I'm quite sure a Schroder 1212 is a more efficent cab than an Ampeg 810.

 

Here is some insight into the B5r from ampeg insider


"The B5R is a relatively newer product, but it is based on the same power amp used in the B4R. It is really half a B4R with a new preamp, so the power section is proven. I wouldn't expect any troubles with it, and I don't know of any.

The power is 500watts @2 ohms, 340 @4, 205 @ 8 and those are continuous ratings, meaning that is what you would measure on the test bench. The unit is solid, you should get at least that power."

Mine never had a problem, never really ran out of steam. There are 340w heads out there with greated percieved volume, but the B5r did a good job of keeping a little in reserve. Seemed to have a lot of headroom for its rated wattage. Also, very flexible distortion if you take the time to learn how to use it. Had it not been for the car wreck, sure it would be rocking along.

The SVT4 will be much lounder, although reports are it does not seem as loud as some other 1000w amps.

On a dollar for dollar basis, I am not sure how much value there is in the Ampeg ss bass amps. I had to get a new rig and went with a light power amp and alembic preamp, which has 2100 watts, far more than i need. Having said that, I loved the plug and play ease of use of the ampeg, and I always sounded great with the band.

 

Those sensitivity ratings are next to worthless. They aren't referenced to frequency (as in at what frequency did they take the measurement) nor do they mention the f3 (half power) and f10 (down 10dB from average) frequencies. If there was an industry wide measurement standard that all manufacturers adhered to one could make a comparison by the specs alone, but since every manufacturer is free to play fast and loose with sensitivity ratings you can't accept any of them at face value.